Cathode ray tubes for television and like purposes



May 29, 1956 E. DAVIES 2,748,305

CATHODE RAY TUBES FOR TELEVISION AND LIKE PURPOSES Filed Dec. 21, 1954 9mm: m Mac 541% GM:

Balm =1 wi ar United States Patent 'CATHODE RAY TUBES 'FBR TELEVISEGN AND LIKE PURPGSES Application December 2-1, 1954, Serial No. 476,667

Claims priority, application Great Britain January 15, 1954 Claims. (Cl. 313- 16) This invention relates to cathode ray tubes for television and like purposes and more specifically to such tubes in which electro-magnetic scanning and focussing coils are provided side by side around the neck of the tube.

It is known in high quality television tubes, i. e., tubes such as those used for television monitors, to supply to the focussing coil fitted round the neck of the tube, correcting wave components derived from the scanning circuits of the tube in order to ensure the maintenance of correct focussing during scanning. The provision of focussing correction during deflection is particularly important, not only in high quality television tubes such as are employed for monitoring purposes but in colour television tubes generally, because, in colour television, defective focussing is liable to cause very adverse efiects in the reproduced pictures and may destroy proper colour registration altogether towards the edges of the screen.

In practice considerable difficulties are experienced with tubes, especially high quality and colour television tubes, having focussing and deflection coils next to one another on the neck of the tube, since leakage fields tend to cause undesired mutual interference between the deflecting and focussing coils. in modern large screen tubes with short necks such interference is apt to be very objectionable, in part because the shortness of the neck means that the coils must be close together, and in part because of the extent of deflection involved in covering a large screen.

Mutual interference between focussing and deflecting coils cannot be satisfactorily eliminated by an ordinary solid magnetic screen between the two coil systems, because such a screen induces losses which are reflected into the circuits of the coils in question and such losses cannot be tolerated in the high efficiency deflecting coil circuits which are necessary if economic scanning is to be effected. The present invention seeks to reduce or substantially eliminate mutual interference between adjacent focussing and deflecting coils without introducing any but very small energy losses.

According to this invention there is provided a television or like cathode ray tube of the kind having focussing and deflection coils next to one another about the neck thereof, wherein at least one magnetic screen is mounted between said coils and said neck, the screen being sub-divided and constructed so as to provide a large number of magnetic paths, said paths together presenting relatively low reluctance to magnetic lines of force running substantially radially with respect to the neck of the tube, and relatively high reluctance to mag netic lines of force running substantially parallel to the axis of the tube.

In a preferred form of construction there is interposed around the neck of the tube and between the focussing and deflecting coils a magnetic screen constituted by a fine wire of flattened toroidal form and made of material having ferro-magnetic properties. The fine wire may be of a suitable resistive alloy, but ordinary soft iron is quite satisfactory. The wire may be wound on an annular former which may be of insulating material and has an inner diameter such as to pass the neck of the tube andan outer diameter large enough to extend radially beyond the coils. The thickness of the former (if used) is made quite small so that the focussing and deflecting coils can be quite close together.

in place of a former of insulating material a former of conductive material, for example, copper, may be used. Although this will introduce a certain amount of eddy current losses are quite small due to the fact that there is ferro-magnetic wire screening on both sides of the copper and the disadvantage of these small losses is more than offset by the improved screening provided by the copper.

in some cases it is possible to use a self supporting winding and dispense altogether with a former.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made to the accompanying drawing, in which:

Fig. l is a schematic representation showing a wound screen, in position round the neck of a tube;

Fig. 2 is a schematic perspective view of the screen removed from the tube; and

Figs. 3 and 4 show stages in the manufacture of a modified form of screen.

Referring to the drawing, 1 is a neck of a television cathode ray tube (shown broken away at both ends of the neck) on which is mounted a focussing coil assembly 2 and a deflecting coil assembly 3. Between these two coil assemblies and acting as a screen to prevent mutual interference is a wound screen 4. The screen 4 consists of an annulus 4a (Fig. 2) which may be either of insulating material or of conductive material such as copper, on which is wound a winding db of fine ferromagnetic and preferably resistive wire, for example, enamel or oxide covered Nichrome wire of 38 S. W. G. The two adjacent ends of the winding may be left open or may be connected together. As will be seen in the figures, the turns of the winding are substantially radial with respect to the neck of the tube and, accordingly, each side of it presents a very large number of radial ferromagnetic paths for lines of force running radially so that there is a relatively low reluctance magnetic path for the fringing fields of both coils to prevent each from coupling materially with the other. Suitable dimensions for a screen 4 as fitted to an existing high quality monitor tube are:

Inches Internal diameter about 1 /2 External diameter about 3% Thickness about ,6

These dimensions are, of course, in no way critical and are chosen to suit the tube and coils in conjunction with which the screen is to be used.

In a modification illustrated in Figs. 3 and 4, the annulus 4a is dispensed with and the winding 41; is wound back and forth in a zigzag so that it occupies a rectangular area as shown in Fig. 3. The winding is then bent round into an annulus as shown in Fig. 4. By using a suitable impregnating material (not shown) the winding of Fig. 4

my be made self-supporting It is not necessary to constitute the screen by actual windings of wire and any equivalent subdivided ferromagnetic structure may be used, for example so-called Wire cloth or woven Wire, cut into an annulus and having wire threads running approximately radially may be used.

While I have described my invention in certain of its preferred embodiments, I realize that modifications may be made, and I desire that it be understood that no limitations upon my invention are intended other than may be imposed by the scope of the appended claims.

I claim:

1. A television or like cathode ray tube of the kind having focusing and deflection coils next to one another about the neck thereof, wherein at least one magnetic screen is mounted between said coils and said neck, the screen being sub-divided and constructed so as to provide a large number of magnetic paths, said paths together presenting relatively low reluctance to magnetic lines of force running substantially radially with respect to the neck of the tube, and relatively high reluctance to magnetic lines of force running substantially parallel to the axis of the tube.

2. A tube as claimed in claim 1, wherein said screen is constituted by a fine wire winding, the wire being from a material having ferro-magnetic properties.

3. A tube as claimed in claim 2, wherein said winding is arranged upon an annular former, the latter having an inner diameter such as to pass said neck, and an outer diameter large enough to extend radially beyond the coils.

4. A tube as claimed in claim 3, wherein said former is of insulating material.

5. A tube as claimed in claim 3, wherein said former is of a conductive material.

6. A tube as claimed in claim 1, wherein said screen comprises a self-supporting winding.

7. A tube as claimed in claim 1, wherein said screen is constituted by a fine wire winding, the wire being made from an electrically resistive material having ferromagnetic properties.

8. A tube as claimed in claim 1, wherein said screen is a sub-divided ferromagnetic structure of the kind hereinbefore set forth, and wherein said structure is of annular shape and is formed so as to provide a plurality of wire threads running approximately radially of said neck.

9. A tube as claimed in claim 1, wherein said screen comprises a winding, the wire in which is wound back and forth in a zigzag to form a rectangular area, said area being deformed in its plane to produce an annulus.

10. A tube as claimed in claim 1, wherein said screen comprises a winding, the wire in which is wound back and forth in a zigzag to form a rectangular area, said area being deformed in its plane to produce an annulus and wherein said annulus is impregnated to increase the rigidity thereof.

References Cited in the file of this patent UNITED STATES PATENTS 

